This invention relates to a CCD (Charge Coupled Device), more particularly to a method for fabricating a charge coupled device which is suitable for protection of active regions from contamination caused when field regions and active regions are formed.
In general, a charge coupled device is an image sensor that receives light in the form of an image from an object and converts the light into electrical signals.
Shown in FIG. 1 is a layout of a conventional charge coupled device.
The conventional charge coupled device includes a plurality of photodiode regions PD of a matrix array for receiving light in the form of an image of the object and converting the received image into a video signal, a plurality of vertical charge coupled device regions VCCD, each formed in the vertical direction between the photodiode regions PD in a matrix array for transmitting charge generated at each of the photodiodes in the vertical direction, a horizontal charge coupled device region HCCD formed on one side of the plurality of vertical charge coupled device regions VCCD for transmitting the charge received from the vertical charge coupled device regions VCCD in the horizontal direction, and a sensing amplifier SA formed at an output terminal of the horizontal charge coupled device region HCCD for producing a video signal from the sensed charges.
And, each of the photodiode regions has a channel stop layer (CST) formed around the photodiode region except for a part for transferring the charge to the vertical charge coupled device region VCCD.
Operation of the charge coupled device having the foregoing construction is as follows.
The photodiode region PD, on receiving light of an image of an object, generates charge corresponding to the received light.
The generated charge is transmitted in the vertical direction through the vertical charge coupled device region VCCD according to a clock signal applied from the outside.
The charge transmitted in the vertical direction is transmitted again in the horizontal direction through the horizontal charge coupled device region HCCD.
The charge transmitted in the horizontal direction, on being sensed by the sensing amplifier SA, is then transmitted to the outside as a video signal.
A conventional method for forming a field oxide film, the photodiode regions PD, and the charge coupled device regions in the charge coupled device will now be explained hereinafter, referring to the attached drawings.
FIGS. 2a to 2h are sections across line A--A' of FIG. 1, showing steps of a method for fabricating the conventional charge coupled device.
As shown in FIG. 2a, a first oxide film 3 and a nitride film 4 are deposited, successively, over the surface of an N type silicon substrate 1, and a P type well 2 is formed in a predetermined part of the N type silicon substrate 1 by ion injection.
Then, a first photosensitive film (not shown) is formed all over the surface of the resultant structure. As shown in FIG. 2b, a field region (FR) and an active region (AR) are then defined therein, the nitride film 4 over the field region is selectively removed, and the first photosensitive film is removed.
Then, a second photosensitive film 5 is deposited again over the surface of the resultant structure and is subjected to patterning with exposure and development processes to expose a field ion injection region in the field region, and field ions (P type) are injected into the exposed field ion injection region of the substrate to a concentration (p.sup.+) higher than that of the well.
Then, the second photosensitive film 5 is removed and, a field oxide film 6 is selectively grown (as shown in FIG. 2c) in the field region FR using a thermal oxidation process with the nitride film 4 serving as a mask.
As shown in FIG. 2d, the nitride film 4 is then removed.
As shown in FIG. 2e, a third photosensitive film 7 is deposited on the surface of the resultant structure, which is then subjected to exposure and development processes to define channel stop CST layers in the P type well 2 in the active region. The CST layers are used as masks for injecting channel stop ions into the P type well to separate pixels of the CCD.
Thereafter, as shown in FIG. 2f, the third photosensitive film 7 as well as the first oxide film 3 are removed.
As shown in FIG. 2g, a second oxide film 8 is formed in the active region as a buffer layer against ion injection for forming the vertical charge coupled device regions VCCD, the horizontal charge coupled device region (not shown in FIGS. 2a-2h), and the photodiode regions PD.
A fourth photosensitive film (not shown) is deposited over the resultant surface which is then subjected to exposure and development processes to define the areas that will become photodiode regions PD. By injecting N type ions into these areas, the photodiode regions PD are formed.
Then, the fourth photosensitive film is removed and, as shown in FIG. 2h, a fifth photosensitive film 9 is deposited over the resultant surface, which is then subjected to exposure and development processes to define the areas of what will become the vertical charge coupled device regions VCCD, and the horizontal charge coupled device region HCCD. By injecting N type ions thereto, the vertical charge coupled device regions VCCD, and the horizontal charge coupled device region (not shown in FIG. 2h) are formed.
Thereafter, by forming gates for transmitting charges, the charge coupled device can be completed.
However, the foregoing conventional method for fabricating the charge coupled device has had following problems.
First, the picture quality can be degraded seriously due to many white defects in the picture caused by OISF (Oxidation Induced Stacking Fault) coming from oxidation during formation of the field oxidation film, which formation requires a long time period of oxidation at an elevated temperature.
Second, the method is subject to possible contamination due to exposure of the active region because the nitride film grown in an initial stage is removed before the field oxidation process for forming the channel stop layers.
Third, since in the prior art method the buried type CCD regions are formed by growing a new oxide film after removal of the oxide film grown in the initial stage, the surface of the substrate is subject to possible contamination by heavy metals.
Fourth, the conventional method is complicated because the separation between the active region and the field region, the separation between the pixels, and the formation of the channel regions are independently performed.